The invention relates to a supplementary joining means for joining sheet metal pieces, particularly those made of light metal. Light metals are defined, for example, in "BROCKHAUS--NATURWISSSENSCHAFTEN UND TECHNIK" [Brockhaus Encyclopedia for Natural Sciences and Technology], Volume 3, Wiesbaden 1983, p. 164 (ISBN 3-7653-0357-7).
The technique of joining two or more metal sheets permanently by means of self-piercing riveting is known. The self-piercing riveting technique is disclosed, for example, in an article by Lothar Budde et al., "Stanznieten ist zukunfstrachtig in der Blechverarbeitung" [Self-Piercing Riveting has a Promising Future in Sheet Metal Processing] from "Bander Bleche Rohre" [Fasteners, Sheet Metals, Pipes], Issue 5, 1991, p. 94 ff.
This publication by Lothar Budde et alia in particular discloses that the supplementary joining means are designed either as solid rivets or as partially-hollow rivets (page 95). The present invention relates to a supplementary joining means in the form of a partially-hollow rivet. According to the description in the Lothar Budde et alia publication, the self-piercing rivet is initially pushed in an uninterrupted joining operation through the sheet on the side of the upper die during the self-piercing operation. For a self-piercing riveting of several layers, the rivet is also pushed through additional sheets and is plastically deformed in the sheet on the die-plate side. In the process, the partially-hollow rivet is provided with its closing head in that a collar is formed in the plastically deformed material of the parts to be riveted together. The pieces punched out during the punching operation in this case remain inside the shank of the partially-hollow rivet.
The joining of metal sheets with self-piercing rivets is known, in particular, from an article by Ortwin Hahn et al., "Stanznieten oberflachenveredelter Stahlbleche" [Self-Piercing Riveting of Surface-Coated Steel Sheets] in BLECH ROHRE PROFILE [Sheet Metal, Pipes and Profiles], Volume 42, 1995, p. 100 ff.
On page 100, the Ortwin Hahn et alia publication initially refers to the fact that during the self-piercing riveting, during the cropping operation, the partially-hollow rivet initially acts as a one-way punch on the sheet facing the upper die. Subsequently, the partially-hollow rivet, together with the sheet on the die-plate side, forms a closing head through plastic deformation. Thus, the partially-hollow rivet consequently must be suitable for initially punching through the sheet metal on the upper die side and must subsequently deform plastically in the sheet metal on the die-plate side.
Pages 103-106 of the publication by Ortwin Hahn et alia in particular contain a description, among other things, of the use of partially-hollow rivets for self-piercing riveting. This description refers to the fact that a partially-hollow rivet with a non-defined rivet base geometry was initially selected, e.g. referred to as "Type I" in the text. A first optimizing stage, referred to as "Type II," teaches the person skilled in the art to provide for a defined rivet base geometry for the partially-hollow rivet. The authors of the Ortwin Hahn et alia publication view the Type III partially-hollow rivet as a second optimizing stage for the partially-hollow rivet, which provides for rounded edges in the region of the rivet head (page 105) in addition to the defined rivet base geometry.
In another publication by Lothar Budde et al., entitled: "Weiterentwicklung der Stanzniettechnik" [Advancements in the Self-Piercing Riveting Method], from BLECH, ROHRE, PROFILE [Sheets, Pipes, Profiles] Issue 39, 1992, p. 310 ff, a partially-hollow rivet with a defined rivet base geometry is disclosed, on page 312, FIG. 7. In order to improve the cutting behavior of the self-piercing rivet, it follows from FIG. 7 as well as the associated text that the authors consider it advantageous if the rivet base is chamfered with an angle of 60.degree.. An inside chamfer with an angle of 60.degree. is viewed as particularly advantageous. This coincides with the information provided in FIG. 13 of the publication by Ortwin Hahn et alia, which shows the first optimizing stage (Type II) and the second optimizing stage (Type III) of the partially-hollow rivet. Both optimizing stages also show partially-hollow rivets with an inside chamfer, provided with the reference .alpha..
The last-named publication by Lothar Budde et alia, from the year 1992, in particular deals with the joining of aluminum sheets. It follows from all previously mentioned publications, and in particular from the last-named publication by Lothar Budde et alia (right column on page 311 there), that self-piercing rivets as well as die-plates are used for joining aluminum sheets, which match those used for joining steel sheets with the same thickness. Partially-hollow rivets made of steel and having an inside chamfer with a chamfer angle of 60.degree. at the rivet base are considered particularly advantageous.
The known steel partially-hollow rivets distinguish themselves through an extremely high tensile strength in the range of approximately 1200 to 1400 N/mm.sup.2. Owing to this high tensile strength, it is also possible to join two sheets by means of self-piercing rivets by using the sharp, ring-shaped cutter of a steel partially-hollow rivet and an adjoining rivet shank with thin wall thickness.
However, the disadvantage when using partially-hollow rivets for joining light metal sheets, particularly aluminum or aluminum alloy sheets, is the much higher inherent weight of the self-piercing rivet made of steel, as compared to the light metals. This higher weight of the self-piercing rivets made of steel is based on the much higher density of ferrous metals as compared to non-ferrous metals, in particular light metals. The use of aluminum sheets is on the rise, primarily in the area of motor vehicle technology and especially for the vehicle body design, in order to reduce the dead weight of the vehicle body. A reduction in the dead weight of the vehicle body will have so-called secondary effects since the weight of other motor vehicle components can also be optimized as a result of the lower vehicle body weight and thus also the lower total weight, e.g. the brakes, the steering mechanism and all its components, and the wheels. Owing to the trend of using light metal sheets, particularly aluminum sheets, in the vehicle body construction in order to reduce the weight, there is an increasingly higher demand for weight-reducing supplementary joining means, meaning partially-hollow rivets made of light metal.
The use of traditional self-piercing rivets made of steel for joining light metal sheets, particularly aluminum sheets, furthermore has disadvantages because of the different electrochemical potentials of steel on the one hand and aluminum on the other. It is unavoidable that these electrochemical potential differences lead to contact corrosion when aluminum sheets are joined with steel self-piercing rivets. In order to avoid contact corrosion, to be sure, it is known from the first-named publication by Lothar Budde et alia, from the year 1991 (page 96 therein), to provide the riveting elements with a surface coating, the application of which is involved and thus expensive. However, from a production technological point of view it is desirable--because it is easier--to be able to use sheets and joining means with similar electrochemical potential, so that the involved and costly surface coating can be omitted.
Finally, when using supplementary joining means and sheets made of different materials, it is more difficult to recycle parts joined in this way. For example, a very involved process is required to separate the steel material from the aluminum material when reprocessing scrap material from vehicle bodies, for which aluminum sheets were used on the one hand and steel self-piercing rivets on the other, before the steel and the aluminum can be separated either for recycling or disposal.